1. Field of the Invention
The present invention relates generally to nuclear reactor facilities, and more particularly to a nuclear reactor pressure vessel having a permanent radiation shield removably attached to the upper external surface of the reactor pressure vessel closure head so as to protect maintenance personnel from exposure to the gamma radiation field originating at the reactor pressure vessel head or its appendages during performance of maintenance operations upon the pressure vessel closure head, such as, for example, detensioning and removal, or insertion and tensioning, of the reactor pressure vessel closure head studs in connection with the performance of refueling operations.
2. Description of the Prior Art
Prior to the performance of refueling operations in connection with, for example, pressurized water reactors (PWRs), the nuclear reactor pressure vessel closure head must first be removed. As is well known, the closure head is conventionally secured to the nuclear reactor pressure vessel by means of suitable, hydraulically actuated tension bolts or studs disposed in a circumferential array about the periphery of the closure head and pressure vessel. In particular, maintenance personnel utilize apparatus known as individual stud tensioners in order to hydraulically detension the bolts or studs whereby the same may be removed and stored, and similarly, upon completion of the refueling operations, the nuclear reactor pressure vessel closure head is replaced upon the reactor pressure vessel and the tension bolts or studs inserted and re-tensioned by means of the maintenance personnel again employing the hydraulic individual stud tensioner apparatus.
The nuclear reactor pressure vessel closure head has mounted thereon in an upstanding manner control rod drive mechanisms (CRDMs) which are provided for controlling the elevational positioning of the reactor control rods relative to the reactor core during performance of their control functions in connection with power plant power requirements. As a result of the power plant operation and the movement of the control rod drive mechanism drive rods relative to the nuclear reactor pressure vessel closure head, as well as the movement of the control rods within the core and the core barrel, highly radioactive deposits, known in the art as crud, tend to accumulate upon the control rod drive mechanisms within the vicinity of the nuclear reactor pressure vessel closure head and within the boundaries, for example, of the control rod drive mechanism cooling baffles which are provided for routing cooling air over the control rod drive mechanism coil stack assemblies in order to cool the same. It may therefore be readily appreciated that the maintenance personnel performing the aforenoted detensioning and removal, or insertion and tensioning, operations in connection with the nuclear reactor pressure vessel closure head studs or bolts during refueling operations are subjected to considerably significant or highlevel dosages of gamma radiation exposure.
In order to address the need for protecting the maintenance personnel from exposure to this highly radioactive gamma radiation field, portable radiation shields have been conventionally employed in such a manner that the same are either suspendingly or upstandingly supported upon the reactor pressure vessel head so as to surround the central hemispherical dome portion of the reactor pressure vessel head upon which the control rod drive mechanisms and their cooling baffles are mounted. While these radiation shields have therefore proven to be somewhat effective in achieving their radiation shielding functions so as to protect the maintenance personnel during the performance of their detensioning or tensioning operations upon the reactor pressure vessel closure head studs, considerable time is required in order to erect and dismantle the portable radiation shields in preparation for the stud detensioning and tensioning operations, respectively. These time requirements also affect the overall time requisites which must be accommodated within the entire time frame necessarily allotted to completion of the entire nuclear reactor refueling operations, and consequently, the provision of such portable radiation shields is not particularly cost effective in connection with the overall operation of the power plant.
In view of the foregoing, other attempts to resolve the conflicting problems of adequately protecting the maintenance personnel from excessive exposure to the highly radioactive gamma radiation field existing within the vicinity of the reactor pressure vessel closure head dome, in a cost-effective manner, have lead to the development of massive pieces of equipment known in the art as multiple stud tensioners. While such equipment or apparatus is therefore capable of performing de-tensioning or tensioning operations simultaneously in connection with a multitude of the reactor pressure vessel closure head studs or bolts, the cost of such equipment or apparatus is virtually prohibitive. Consequently, while such apparatus has therefore considerably reduced, or virtually eliminated, the need for the aforenoted radiation shielding in view of the fact that the de-tensioning or tensioning operations may be rapidly performed within a relatively short period of time, and therefore the time period within which the maintenance personnel are exposed to the existing radiation field present within the vicinity of the reactor pressure vessel head dome has been minimized, such apparatus is also not seen to be a cost-effective means for achieving the stud de-tensioning or tensioning maintenance operations while simultaneously protecting the maintenance personnel from exposure to the existing reactor pressure vessel closure head radiation field.
Accordingly, it is an object of the present invention to provide a new and improved nuclear reactor pressure vessel radiation shield.
Another object of the present invention is to provide a new and improved nuclear reactor pressure vessel radiation shield which overcomes the various drawbacks and operational disadvantages of prior art nuclear reactor pressure vessel radiation shields.
Yet another object of the present invention is to provide a new and improved nuclear reactor pressure vessel radiation shield which is economically cost-effective to fabricate, install, utilize, and maintain.
Still another object of the present invention is to provide a new and improved nuclear reattor pressure vessel radiation shield which permanently, but removably, attaches to the exterior surface of the pressure vessel closure head so as to peripherally surround the control rod drive mechanism (CRDM) assemblies and their assoociated cooling baffles.
Yet still another object of the present invention is to provide a new and improved nuclear reactor pressure vessel radiation shield which permanently, but removably, attaches to the exterior surface of the pressure vessel closure head so as to peripherally surround the control rod drive mechanism (CRDM) assemblies and their associated cooling baffles, yet be disposed radially within the circumferential array of tension bolts or studs disposed at the periphery of the pressure vessel closure head so as to protect maintenance personnel servicing the closure head tension bolts or studs from the radiation emitted within the region of the CRDM assemblies.
Still yet another object of the present invention is to provide a new and improved nuclear reactor pressure vessel radiation shield which permanently, but removably, attaches to the exterior surface of the pressure vessel closure head so as to readily facilitate the implementation of refueling operations without delaying the initiation of the refueling operations.
A further object of the present invention is to provide a new and improved nuclear reactor pressure vessel radiation shield which permanently, but removably, attaches to the exterior surface of the pressure vessel closure head which readily provides necessary access to various auxiliary equipment or system components located upon the closure head.
A yet further object of the present invention is to provide a new and improved nuclear reactor pressure vessel radiation shield which permanently attaches to the exterior surface of the pressure vessel closure head yet which is removable therefrom so as to provide necessary access to large areas of the closure head and equipment or systems operatively associated therewith when such equipment or systems require maintenance, service, replacement, or the like.
A still further object of the present invention is to provide a new and improved nuclear reactor pressure vessel radiation shield which is permanently, but removably, attached to the exterior surface of the pressure vessel closure head in such a manner as to accommodate existing structural components and equipment which are operatively associated with the closure head in a structurally integrated system.
A yet still further object of the present invention is to provide a new and improved nuclear reactor pressure vessel radiation shield which is permanently, yet removably, attached to the exterior surface of the pressure vessel closure head so as to be removable along with the pressure vessel closure head, if so desired, when the closure head is in fact removed from the nuclear reactor pressure vessel.
A still yet further object of the present invention is to provide a new and improved nuclear reactor pressure vessel radittion shield which is permanently, yet removably, attached to the exterior surface of the pressure vessel closure head which can accommodate thermal expansion of the closure head while the radiation shield of the present invention is attached to the closure head.
An additional object of the present invention is to provide a new and improved nuclear reactor pressure vessel radiation shield which is readily adaptable to existing nuclear reactor power plant facilities in a retrofittable manner.